Historically, power-sharing arrangements in Russia have ended in catastrophe. Ever since the 16th century regency of Boris Godunov, which led to Russia's fabled “time of troubles”, conflict at the top has tended to trickle down.
Examples from the 20th century are numerous, from the doomed provisional government of 1917, which ushered in the Bolshevik revolution, to the conflict between Boris Yeltsin and his vice president Aleksandr Rutskoi, which led to armed confrontation in the streets of Moscow as recently as 1993.
Against this backdrop, the current experiment in power-sharing between President Dmitry Medvedev and his mentor, Prime Minister Vladimir Putin, seems risky, but so far appears to be working.